Solid state light emitting devices such as light emitting diodes (LEDs) and solid state lasers are used in a wide range of applications from conventional lighting systems to optical communication systems. LED-based lighting systems are increasingly popular due to the low energy consumption and long life length of LEDs in comparison to incandescent light sources.
Even though LEDs already are more energy efficient than conventional light bulbs, it is desirable to further improve the energy efficiency of the LED. However, extraction of light generated in an LED is limited by trapping of light within the device due to total internal reflection of light reaching the interface between the LED and a surrounding material at angles exceeding the Brewster angle. In particular, extraction efficiency is limited by the difference in refractive index between the LED and the surrounding material. As an example, in a GaN-based LED terminated in a flat GaN surface, only about 4% of the generated light is actually emitted.
There have been different attempts aimed at improving the extraction efficiency, such as for example forming a GaN LED on a textured sapphire substrate in order to form an interface where the amount of total internal reflection is reduced. The extraction efficiency may also be improved by roughening the light extraction surface using various surface treatment methods. However, surface roughening suffers from a low degree of control over the resulting surface properties and chip-scale LED processing is complicated.
US2011/0263128 suggests using selective wet etching to form a pattern comprising protrusions or indentations in a GaN layer of a light emitting diode in order to influence the extraction efficiency and/or collimation of light emitted by the LED. US2011/0263128 also discloses patterning the surface of a sapphire substrate through selective wet etching.
Even though the extraction efficiency may be improved through the aforementioned approach, it is still desirable to further improve the extraction efficiency in a light emitting diode.